Modern integrated circuits (ICs) are composed of multiple layers of conductors and substrate materials, such as insulators and semiconductors. Inspecting and editing a circuit or other hidden interior feature in an IC requires navigating to the target area and milling through one or more of the multiple layers of substrate material. Circuit Edit (CE) reduces IC development costs by reducing the number of mask sets that are required during the design-debug phase, and speeds overall time-to-market.
Most CE activities today are performed with Focused Ion Beam (FIB) systems, which are commonly used to mill away a substrate material to expose hidden features and also deposit materials with high precision. These capabilities can be used to cut and connect circuitry within a device, as well as to create probe points for electrical test. Applications include validating design changes, debugging and optimizing devices in production, and prototyping new devices without costly and time-consuming mask set fabrication.
Typically material removal in FIB systems is accomplished by using beams of relatively large ions to physically sputter away the substrate material. Most FIB systems use gallium ions (Ga+) produced by a Liquid Metal Ion Source (LMIS) because such sources are easy to fabricate, operate at room temperature, and are reliable, long lived, and stable. In addition, chemical agents can be introduced onto the work piece during FIB processing, to favorably manipulate the milling rates of selected materials. The use of chemical agents to enhance or suppress FIB milling rates is generally referred to as “Gas-Assisted Etching” (GAE).
Because polyimide (PI) is a common encapsulating material on IC package devices, it is often necessary to remove a portion of a polyimide layer during circuit edit or failure analysis. On common method of polyimide removal is by etching the polyimide layer with a Ga+ FIB in the presence of water vapor. The water acts as an etch-assisting gas and is known to increase the polyimide etch rate by a factor of 5 to 10 times the default milling rate (defined as the milling rate using the FIB without an etch-assisting gas). The use of water vapor as an etch-assisting gas for organic (carbon containing) compounds such as polyimide is described in U.S. Pat. No. 5,958,799 to Russell et al., for “Method for water vapor enhanced charged-particle-beam machining” (Sep. 28, 1999), which is hereby incorporated by reference.
While the Ga+ FIB has been the most common type of FIB used in IC manufacturing for decades, plasma FIB instruments using inert ions such as xenon ions (Xe+) offer a number of significant advantages to the traditional Ga+ FIB using a liquid metal ion source. For example, a plasma FIB provides beam currents which are 20 to 100× the beam currents used in traditional gallium-based FIBs, which results in a tremendous increase in material removal rates. Also, plasma FIBs using inert ions do not result in problematic ion contamination such as that caused by Ga+ implantation.
One significant disadvantage of using a plasma FIB such as a Xe+ plasma FIB for circuit edit or failure analysis on IC package devices encapsulated by polyimide or other similar organic films is that water vapor does not appear to act as an etch-assisting gas for polyimide when used with a X+ plasma FIB. Although other ex-situ (outside the FIB vacuum chamber) methods of polyimide removal are known, including lasers and plasma etching tools, no effective in-situ methodology for Xe+ FIB tools is known in the prior art.
Accordingly, what is needed is method for more rapid and efficient in-situ etching of Polyimide and other organic films using a Xe+ plasma beam.